Subsurface remediation of sites consisting of low permeability (tight) geological formations has been challenging tasks for environmental professionals. These soils usually refer to deposits consisting of consolidated clay material where acceptable vapor and groundwater flow movements cannot be achieved. As a result, corrective measures suitable to remedy tight soils are few in number and continue to have significant shortcomings.
Most commonly implemented groundwater remedial options are based on extraction of contaminated water for treatment at the surface or within the well, or injection of treatment chemical compounds to contaminants location. Unfortunately, neither of the currently existing processes is effective in low permeability formations.
For extraction methods to be effective, the remediation site must present high hydraulic conductivity that allows for movement of adequate volumes of groundwater. In most low permeability formations, groundwater extraction from a well may not exceed a fraction of a gallon per minute. Measures to enhance groundwater flow, including high vacuum enhanced extraction, have been attempted. These measures can be effective in increasing groundwater extraction rate. However, these measures when effective will result in the need to remedy and dispose of a large volume of water. This can be costly and maintenance demanding.
Other difficulties and limitations are presented by the groundwater extraction method approach to remediation of groundwater. For example, when the water table is deeper than 25 feet below the surface of the ground, the use of high vacuum extraction alone is not effective for pollution remediation. Such cases require the use of expensive and maintenance demanding liquid ring, high vacuum blowers to exert high vacuum forces in the well and on groundwater surrounding the well.
Another limitation of the groundwater extraction methods and devices is their radius of influence—the distance around the well on which the well has its beneficial effect—is usually less than 15 feet and may be as small as 3 feet. The prior art groundwater extraction devices and method require that a large number of wells be emplaced to effectively treat the polluted ground water. In cases in which it is necessary to actually remove the extracted groundwater from the well for treatment on the surface, the remediation costs are increased. Further, the prior art groundwater extraction methods present no suitable methods for treatment of polluted groundwater that is contained in tight or low permeability ground formations. The application of high vacuum in wells may result in creating fractures in the soil surrounding the well thereby presenting preferential pathways for vapor and groundwater movement toward the well resulting in treatment of only limited portions of the groundwater near the well.
Injection based groundwater remedies are based on forcing treatment compounds under high pressure into the ground to reach contaminated groundwater in-situ. A number of difficulties are associated with injection-based technologies. These include the potential for spreading contaminants from polluted areas into clean areas; the potential for injected chemicals to remain localized when injected into fractured rock formations; the consumption of significant volumes of the injected chemicals by natural soil constituents; and the ineffectiveness of such injection methods when the contaminants are present at low concentration.
Injection methods also may require high injection forces which are likely to result in destruction of soil matrix resulting in additional soil consolidation, which further limits the potential of movement of injected fluids. In addition, the high pressure may result in soil heaving and the dispersal of injected materials to the upper layers of soil into the vadose zone. Further, the expense of injection methods can be very high for situations in which several injection applications are needed. Large volumes of chemicals may be necessary to achieve acceptable reduction, and as injection methods tend to provide a small radius of influence, an extensive array of injection points may be necessary.
Therefore, as currently available tight formation, or low permeability formation, remedial measures are of limited effectiveness at most sites, it would be of great benefit if an in-well treatment technology, capable of exerting forces adequate to extract groundwater into the well and inject it back in the same well after treatment were available.
The present methods of dealing with groundwater remediation also include circulation well technology. The circulation technology method combines in-situ air stripping, air-sparging, soil vapor extraction and enhanced bioremediation oxidation in a wellhead system. The system is designed for inclusion in a well of at least four inches in diameter and is highly cost effective when compared with other, stand-alone remediation technologies. An example of this type of method is shown in U.S. Pat. No. 6,557,633, to Abouodah, the specification of which is incorporated herein by reference.
The air-sparging component results in lifting the water table. This lifting of the water in the well causes a net reduction in head at the well location, which results in water flowing toward the well. Vacuum pressure (the vapor extraction component) is applied atop of the well point to extract vapor from the subsurface. The negative pressure from vacuum extraction results in water suction that creates additional water lifting (mounding) and a net lower gradient. This further enlarges the radius of influence of the well.
A submersible pump is placed at the bottom of the well to recirculate water to a point above the mounded groundwater downward spray through a spray head. The water cascades down the interior of the well and counter flow to the vapor extraction similar to what occurs in an air-stripping tower. Enhanced stripping via air-sparging near the bottom of the well will occur simultaneously. In essence, the well will act as a subsurface air-stripping tower. In addition to the air stripping effected by the pumping/cascading, a portion of the pumped, stripped, highly aerated water will flow down well annulus and over the mounded groundwater back in to the aquifer, thus creating a circulation zone surrounding the well. These combined effects, in concert will accelerate and further enhance cleanup. Effects of the different forces on the groundwater table in relation to the wellhead technology are shown in FIG. 1.
In summary, contaminants are stripped from water as a result of the combined effects in-well air stripping and in-well air sparging. The radius of influence, or the groundwater cleaning zone, will be created by a combination of negative gradient as a result of air-sparging, additional, negative gradient resulting from the application of vacuum extraction, and subsurface water circulation induced by a submersible pump. All of these different components can be integrated and installed in a 4-inch groundwater well. Cost of this technology is in the range of the cost of air-sparging technology alone; since the costs of added pump and piping will be compensated for by the elimination of a separate vapor extraction point and associated trenching and construction.
Previous known circulating well technologies include the NoVOCTM and Underduck-Verdampfer-Brunnen (UVB) technologies. In the NOVOCTM technology, a blower introduces air to produce bubbles in a sparging well. The well is equipped with a deflector plate that separates two screens. When the sparged air encounters the deflector plate, the bubbles break, re-combine and then re-infiltrate the vadose zone to be extracted through the upper screen. With the UVB technology, airlift pumping occurs in response to negative pressure induced at the wellhead by a blower. Vacuum draws water into the well through the lower screen. As a result, air is introduced through a diffuser plate located within the upper, screened section. The air bubbles provide airlift pump effect that moves water up in the well. A submersible pump is installed to insure that water flows from bottom to top. A stripping reactor consisting of fluted and channeled columns is installed to facilitate transfer of volatile compounds to the gaseous phase.